1. Field of the Invention
The present invention relates to a displacement detecting device for accurately detecting displacement of a surface-to-be-measured by a non-contact sensor which uses light.
2. Description of the Related Art
Conventionally, displacement detecting devices are widely used for measuring displacement and shape of a surface-to-be-measured.
Among these displacement detecting devices, there is a displacement detecting device that detects the displacement of the surface-to-be-measured in a non-contact manner by using light. In such displacement detecting device, for example, the light emitted from a light source is condensed on the surface-to-be-measured by an objective lens. Further, the light reflected by the surface-to-be-measured is condensed by an astigmatic optical element so that astigmatism is generated therein, and the light having the astigmatism generated therein is incident on a light receiving element.
Based on the intensity of the light received by the light receiving element, a focus error signal is generated by an astigmatism method, and a servo is operated to displace the objective lens so that the focus position of the condensed light is located on the surface-to-be-measured.
Further, a linear scale is integrally attached to the objective lens through a connecting member, so that the linear scale also moves due to being interlocked with the displacement of the objective lens. The scale of the moving linear scale is read by a detection head fixed to a fixed point, and thereby the displacement in the height direction of the surface-to-be-measured is detected.
However, in such displacement detecting device, since linearity of the focus error signal itself is bad, high detection accuracy can not be obtained.
To solve this problem, a method for correcting the focus error signal of the non-contact sensor by using a correction table is proposed in Japanese Unexamined Patent Application Publication No. H05-89480.
In the displacement detecting device disclosed in Japanese Unexamined Patent Application Publication No. H05-89480, in order to improve the accuracy of the displacement detection, NA (Numerical Aperture) of the objective lens is set large, and therefore the beam diameter condensed on the surface-to-be-measured reduced. For example, when the beam diameter formed on the surface-to-be-measured is about 2 μm, the detection accuracy of the linear scale will be in a range between several nm and several hundreds nm.